Output circuit, and data driver and display device using the same

ABSTRACT

An output circuit includes a connection switch and an operation unit. The connection switch includes first to third terminals for receiving first to third voltages, respectively, selects and outputs the first voltage or the second voltage or the third voltage to each of the first to seventh intermediate terminals, including selection of the same voltage for a plurality of the intermediate terminals. The connection switch switches assignment of the first to third voltages to the first to seventh intermediate terminals responsive to a connection switching signal. The operation unit receives the voltages assigned to the first to seventh intermediate terminals and outputs to an output terminal a voltage obtained by performing a predetermined operation on the voltages supplied to the first to seventh intermediate terminals.

REFERENCE TO RELATED APPLICATION

This application is a Divisional application of U.S. patent applicationSer. No. 11/979,714, filed on Nov. 7, 2007, now U.S. Pat. No. ______tbd.

This application is based upon and claims the benefit of the priority ofJapanese patent application No. 2006-302956, filed on Nov. 8, 2006, thedisclosure of which is incorporated herein in its entirety by referencethereto.

FIELD OF THE INVENTION

The present invention relates to an output circuit, and a data driverand a display device using the output circuit.

BACKGROUND OF THE INVENTION

Recently, as display devices, liquid crystal display (LCD) devices thatfeature thinness, lightweight, lower power consumption have beenwidespread used, and have been extensively utilized as display units ofmobile devices including portable telephone apparatuses (such as mobilephones or cellular phones), PDAs (personal digital assistants), andnotebook PCs. Recently, however, a technology for supporting a largerscreen and a moving image of the liquid crystal display devices has beendeveloped. Then, realization of tabletop large-screen display devicesand tabletop large-screen liquid crystal TVs as well as display devicesand TVs for mobile use have become possible. As these liquid crystaldisplay devices, an active matrix driving system liquid crystal displaydevice capable of performing high-definition display is employed.

First, referring to FIG. 25, a typical configuration of the activematrix driving system liquid crystal display device will be outlined.FIG. 25 schematically shows a main configuration connected to one pixelin a liquid crystal display unit, using an equivalent circuit.

Generally, a display unit 960 of the active matrix driving liquidcrystal display device includes a semiconductor substrate, an opposingsubstrate, and a liquid crystal sealed in between these two substratesby opposing these two substrates. On the semiconductor substrate,transparent pixel electrodes 964 and thin-film transistors (TFTs) 963are arranged in a matrix form (of 1280×3 pixel rows×1024 pixel columnsin the case of a color SXGA panel, for example). One transparentelectrode 966 is formed on an entire surface of the opposing substrate.

Turning on and off of a TFT 963 having a switching function iscontrolled by a scan signal. When the TFT 963 is turned on, a gray scalevoltage corresponding to a video signal is applied to a correspondingpixel electrode 964. Transmittance of the liquid crystal is changed by apotential difference between each pixel electrode 964 and the opposingsubstrate electrode 966, and the potential difference is held by aliquid crystal capacitance 965 for a certain period, thereby displayingan image.

On the semiconductor substrate, data lines 962 and scan lines 961 arewired in the form of a grid (in which 1280×3 data lines and 1024 scanlines are arranged in the case of the color SXGA panel described above).A data line 962 sends a plurality of level voltages (gray scalevoltages) applied to each pixel electrode 964, and a scan line 961 sendsthe scan signal. Due to a capacitance produced at an intersectionbetween each of the scan lines 961 and each of the data lines 962 and aliquid crystal capacitance sandwiched between the semiconductorsubstrate and the opposing substrate, the scan lines 961 and the datalines 962 have become a large capacitive load.

The scan signal is supplied to a scan line 961 from a gate driver 970,and a gray scale voltage is supplied to each pixel electrode 964 from adata driver 980 through a data line 962.

Rewriting of data of one screen is usually performed in one frame period(of approximately 1/60 seconds). Data is successively selected everypixel row (every line) by each scan line, and a gray scale voltage issupplied from each data line within a selection period.

While the gate driver 970 should supply the scan signal of at least twovalues, the data driver 980 needs to drive a data line by the gray scalevoltage of multi-valued levels corresponding to the number of grayscales. For this reason, the data driver 980 includes a decoder thatconverts video data to a gray scale voltage signal and adigital-to-analog converter circuit (DAC) formed of an operationalamplifier that amplifies a voltage of the gray scale voltage signal, foroutput to a corresponding data line 962.

Recently, image quality of liquid crystal display devices has beenimproved (or the number of colors used in the liquid crystal displaydevices has been increased). There has been a growing demand for atleast 260 thousand colors (video data of six bits for each of colors ofR, G, B) and 16,770 thousand colors (video data of eight bits for eachof the colors of R, G, B), and 1,074,000 thousand colors (video data of10 bits for each of the colors of R, G, B) or more.

For this reason, for a data driver that outputs a gray scale voltagecorresponding to multi-bit video data, a voltage output with highaccuracy is demanded. In addition, the number of devices in a circuitportion that handles the video data has increased, and the chip area ofa data driver LSI has increased, which has become a factor in resultingin high cost. This problem will be described below in detail.

FIG. 26 is a diagram showing a configuration of the data driver 980 inFIG. 25, and showing a main portion of the data driver 980 using blocks.Referring to FIG. 26, the data driver 980 is configured by including alatch address selector 981, a latch 982, a reference voltage generationcircuit (gray scale voltage generation circuit) 983, decoders 984, andamplifiers (buffer circuits) 985.

Based on a clock signal CLK, the latch address selector 981 determines adata latch timing. The latch 982 latches digital video data based on thetiming determined by the latch address selector 981, and outputs thedata to each of the decoders 984 in response to an STB signal (strobesignal) in unison. The reference voltage generation circuit 983generates reference voltages (gray scale voltages) the number of whichis the number of gray scales corresponding to the video data. Eachdecoder 984 selects one of the reference voltages corresponding to inputdata and outputs the selected reference voltage. Each of the amplifiers985 receives the gray scale voltage output from a corresponding one ofthe decoders 984 and performs current amplification, for output as anoutput voltage Vout.

When 6-bit video data is input, the number of gray scales is 64. Then,the reference voltage generation circuit 983 generates the referencevoltages (gray scale voltages) of 64 levels. The decoder 984 has acircuit configuration in which the decoder 984 selects one gray scalevoltage from among the gray scale voltages of 64 levels.

On the other hand, when 8-bit video data is input, the number of grayscales becomes 256. Then, the reference voltage generation circuit 983generates the reference voltages (gray scale voltages) of 256 levels.The decoder 984 has a circuit configuration in which the decoder 984selects one gray scale voltage from among the gray scale voltages of 256levels.

On the other hand, when 10-bit video data is input, the number of grayscales becomes 1024. Then, the reference voltage generation circuit 983generates the reference voltages (gray scale voltages) of 1024 levels.The decoder 984 has a circuit configuration in which the decoder 984selects one gray scale voltage from among the gray scale voltages of1024 levels.

As described above, when the number of bits of video data is increased,a circuit size of each of the reference voltage generation circuit 983and the decoders 984 increases. When the video data is increased fromsix bits to eight bits, the circuit size becomes not less than fourtimes of that for six bits. When the video data is increased from sixbits to 10 bits, the circuit size becomes not less than 16 times of thatfor six bits.

Accordingly, the chip area of the data driver LSI is increased, thusleading to high cost due to an increase in bits of the video data.

On contrast therewith, as a technology for restraining an increase inthe chip area of the data driver LSI even if the number of bits of videodata is increased, a description in U.S. Pat. No. 6,246,351 (PatentDocument 1) is referred to.

FIG. 27 is a diagram for explaining the technology disclosed in PatentDocument 1 (corresponding to FIG. 2 in Patent Document 1). Referring toFIG. 27, a DAC in FIG. 27 is formed of a string DAC unit (decoder unit)4001 and an interpolating amplifier unit 4100. The string DAC unit(decoder unit) 4001 includes a string constituted from a set ofresistance elements R000 to R255 and switches S000 to S255 which selecta set of voltages at both ends of a resistance. The interpolatingamplifier unit 4100 includes a differential amplifier including aplurality of differential pairs of the same polarity and switches 4004for selectively receiving voltages supplied to two input terminals 4002and 4003 to non-inverting inputs of the differential amplifier.

In the string DAC unit 4001, by the switches S000 to S255 controlled byhigher-order M bits of digital data, two voltages at both ends of oneresistance among the resistance elements R000 to R255 of the resistorstring are selected. Then, the selected voltages are supplied to inputterminals 4002 and 4003 of the interpolating amplifier unit 4100,respectively.

The two voltages selected by the switches are limited to the voltages atboth ends of the one resistance among the resistance elements R000 toR255 of the resistor string. Voltages at both ends of a plurality of theresistance elements are not selected, or the same voltage is notselected.

The interpolating amplifier unit 4100 selectively inputs voltages V1 andV2 supplied to the input terminals 4002 and 4003, respectively, tonon-inverting inputs 4111, 4121, 4131, and 4141 by the switches 4004controlled by lower-order N bits of the digital data, and can output avoltage obtained by internally dividing the voltages V1 and V2 at anarbitrary ratio, according to a ratio between the number of the voltagesV1 and the number of the voltages V2. The non-inverting input 4111 of adifferential pair 4110 is connected to the input terminal 4002. Anoutput terminal Vout is feedback connected to an inverting input 4112 ofthe differential pair 4110, an inverting input 4122 of a differentialpair 4120, an inverting input 4232 of a differential pair 4130, and aninverting input 4142 of a differential pair 4140.

Since the four differential pairs (4110, 4120, 4130, 4140) are providedin FIG. 27, voltages obtained by internally dividing the voltage V1 atthe terminal 4002 and the voltage V2 at the terminal 4003 at a ratio of1:3, 1:1, and 3:1 and four voltages of a voltage Vin2 can be output, dueto an LSB (Least Significant Bit).

Accordingly, for the number of voltage levels to be output, the numberof input voltage levels can be reduced to even the inverse of the numberof differential pairs. For this reason, the number of power supply linesof the string DAC unit and the area of the string DAC unit can bereduced.

A technology for implementing an increase in accuracy of an outputvoltage in addition to area saving of a data driver, a configuration inFIG. 15 of JP Patent Kokai Publication No. JP-P-2001-343948A (PatentDocument 2), for example, can be pointed out.

FIG. 28 shows an example of a configuration of an amplifier circuit inan output unit of a data driver corresponding to the configuration inFIG. 15 in the document described above. Referring to FIG. 28, theamplifier circuit is configured by including an amplifier 85-1 and aswitch circuit 42. The amplifier 85-1 is the amplifier capable ofoutputting a voltage obtained by 1:1 internal division of voltages inputto terminals IN1 and IN2 to a terminal OUT. Since the number of inputpower supply lines can be reduced to a half of the number of voltagelevels to be output, the area of a DAC portion can be reduced.Connection among each of differential input terminals for the amplifier,each of the terminals IN1 and IN2, and output terminal OUT is controlledby the switch circuit 42, and can assume the following four states.

(1) First State:

Terminals Q12 and Q13 are connected to the terminals IN1 and IN2,respectively. Terminals Q11 and Q14 are connected to the output terminalOUT.

(2) Second State:

The terminals Q12 and Q13 are connected to the terminals IN2 and IN1,respectively. The terminals Q11 and Q14 are connected to the outputterminal OUT.

(3) Third State:

The terminals Q11 and Q14 are connected to the terminals IN1 and IN2,respectively. The terminals Q12 and Q13 are connected to the outputterminal OUT.

(4) Fourth State:

The terminals Q11 and Q14 are connected to the terminals IN2 and IN1,respectively. The terminals Q12 and Q13 are connected to the outputterminal OUT.

Then, the four states described above are switched according to apredetermined cycle. An output offset caused by threshold valuevariations of transistors forming the amplifier 85-1 is time averaged,and cancelled.

Accordingly, by using the configuration in FIG. 28, the area of the DACportion can be reduced. An output accuracy of the amplifier 85-1 canalso be improved.

However, it is inferred that, since two gray scale voltages are suppliedto non-inverting input terminals in the third and fourth states in theconfiguration in FIG. 28, the data driver cannot output a desiredvoltage properly.

In the case of FIG. 28, it is inferred that when the first and secondstates are switched, there is a certain effect on cancellation of theoutput offset.

PATENT DOCUMENT 1

-   U.S. Pat. No. 6,246,351 (FIG. 2)

PATENT DOCUMENT 2

-   JP Patent Kokai Publication No. JP-P-2001-343948A (FIG. 15)

SUMMARY OF THE DISCLOSURE

The entire disclosure of Patent Documents 1 and 2 are incorporatedherein by reference thereto.

As described above, according to the configuration shown in FIG. 27(shown in Patent Document 1), the more the number of the differentialpairs is increased, the more an effect of reducing the area of thestring DAC unit can be improved. However, the improvement in accuracyachieved by state switching as shown in FIG. 28 cannot be implemented.

When the accuracy of the amplifier is deteriorated as described above,variations in output voltages of respective outputs of the data driverwill arise. As a result, a defect such as display unevenness or avertical streak will appear on a liquid crystal display screen.

On the other hand, according to the configuration (Patent Document 2)shown in FIG. 28, by switching the four connection states, a highlyaccurate output voltage can be obtained. However, this connectionswitching can be applied only to an amplifier having two differentialpairs as shown in the amplifier 85-1 in FIG. 28, and cannot be appliedto the amplifier having the arbitrary number of differential pairs asshown in FIG. 27.

According to the configuration (Patent Document 2) shown in FIG. 28, theimprovement in accuracy can be realized. However, area saving of the DACportion cannot be implemented so much as the configuration shown in FIG.27.

Accordingly, when an output circuit in FIG. 28 is used for a datadriver, variations in output voltages of respective outputs of the datadriver can be restrained, and image quality of a liquid crystal displayscreen can be improved. However, there is a problem that cost of thedata driver becomes higher than in a case where a data driver using anoutput circuit in FIG. 27 is configured.

As an approach to solving these problems or simultaneously realizingarea saving and improvement in accuracy of a data driver, application ofswitching between the two states to FIG. 27 may be conceived. Theswitching between the two states refers to switching between the firstand third states, or switching between the second state and the fourthstate, for example. The switching between the two states meansinterchange between an inverting input side and a non-inverting inputside in the differential amplifier in FIG. 28.

According to this approach, an error caused by mismatching betweencorresponding devices on the inverting input side and the non-invertinginput side of the interpolating amplifier unit 4100 in FIG. 27(mismatching between transistors on inverting and non-inverting sides ofthe differential pair 4110 or a load circuit (current mirror) 4150) canbe cancelled by the connection switching.

However, mismatching among the differential pairs (e.g. mismatchingbetween an inverting input side transistor of the differential pair 4110and an inverting input side transistor of the differential pair 4120 andmismatching between current sources) cannot be cancelled.

As described above, in order to enhance the effect of reducing the areaof the data driver, it is effective to increase the number of thedifferential pairs in the interpolating amplifier unit 4100.

However, the more the number of the differential pairs increases, moreheavily affects the mismatching among the differential pairs on theaccuracy of an output voltage. Thus, when the number of outputs of thisamplifier is increased, only switching between inverting inputs andnon-inverting inputs may not reduce mutual variations of output voltagesof the amplifier.

Further, as another approach, application of an approach to switchingall possible states in FIG. 27 may be conceived.

In this approach, connection states corresponding to all combinations ofinputs and outputs are switched. Thus, theoretically, mismatchingbetween the non-inverting input side and the inverting input side andthe mismatching among the differential pairs can be all cancelled.

However, when the interpolating amplifier unit 4100 in FIG. 27 has Ndifferential pairs, all the states that can be assumed reaches thecombinations of as large as N!×2.

When there are two differential pairs, for example, the number of allthe combinations is four as in FIG. 28. When the number of differentialpairs is three, the number of all the combinations is 3!×2=12. When thenumber of differential pairs is four, the number of all the combinationsis 4!×2=48. When the number of differential pairs is five, the number ofall the combinations is 5!×2=240. When the number of differential pairsis increased, the number of combinations of possible states will berapidly increased.

When this amplifier is employed for a display device, switching ofdozens or hundreds of connecting ways in order to cancel all mismatchingextremely prolongs the time necessary for canceling an output voltageerror due to the mismatching between devices.

For this reason, in this connection switching approach, an outputvoltage error in each connection state may be recognized as a flicker tohuman eyes. Then, after all, image quality of a liquid crystal displaydevice will be degraded.

Further, in order to implement switching of dozens or hundreds ofconnections, a greater number of switches need to be provided for theamplifier in FIG. 27. Thus, there is also a problem that the area of theswitches becomes large, and an area saving effect is reduced.

Accordingly, it is an object of the present invention to provide anoutput circuit in which mismatching between an inverting input side anda non-inverting input side of an amplifier having three or moredifferential pairs and mismatching among the differential pairs can becanceled by switching of the small number of connection states, therebyallowing area saving and reduction of mutual voltage variations amongoutputs.

It is another object of the present invention to provide a low-cost andhighly accurate data driver with a saved area by using the outputcircuit described above. Further, another object of the presentinvention is to provide a display device including a data driver inwhich cost reduction, reduction of the width of a frame, and higherimage quality are implemented.

An output circuit according to an aspect of the present inventionincludes:

-   -   a connection switch that includes first and second terminals for        receiving a first voltage and a second voltage, respectively,        and first to third intermediate terminals and that selects and        supplies the first voltage or the second voltage to each of said        first to third intermediate terminals, including selection of        the same voltage for a plurality of the intermediate terminals,        said connection switch switching assignment of the first voltage        and the second voltage to said first to third intermediate        terminals responsive to a connection switching signal; and    -   an operation unit that receives the voltages supplied to said        first to third intermediate terminals, and outputs to an output        terminal of said output circuit a voltage obtained by performing        a predetermined operation on the voltages supplied to said first        to third intermediate terminals.

In the present invention, the connection switch performs switchingbetween a first connection state and a second connection stateresponsive to the connection switching signal;

-   -   in the first connection state, the connection switch outputs the        first voltage, the second voltage, and the second voltage to the        first, second, and third intermediate terminals, respectively;    -   in the second connection state, the connection switch outputs        the second voltage, the second voltage, and the first voltage to        the first, second, third intermediate terminals, respectively;        and    -   the operation unit outputs to the output terminal an average        voltage of the voltages supplied to the first to third        intermediate terminals.

In the connection switch of the present invention, a switch is connectedbetween a first terminal with the first voltage applied thereto and thefirst intermediate terminal, and a switch is connected between a secondterminal with the second voltage applied thereto and the thirdintermediate terminal, the switches being controlled by the connectionswitching signal;

-   -   a switch is connected between the first terminal and the third        intermediate terminal, and a switch is connected between the        second terminal and the first intermediate terminal, the        switches being controlled by a complementary signal of the        connection switching signal; and    -   the second terminal is connected to the second intermediate        terminal, and the second voltage is output to the second        intermediate terminal irrespective of a state of the connection        switching signal.

An output circuit according to another aspect of the present inventionincludes:

-   -   a connection switch that includes first to third terminals for        receiving first to third voltages, respectively, and first to        seventh intermediate terminals, and that selects and supplies        the first voltage, the second voltage, or the third voltage to        each of said first to seventh intermediate terminals including        selection of the same voltage for a plurality of the        intermediate terminals, said connection switch switching        assignment of the first to third voltages to said first to        seventh intermediate terminals responsive to a connection        switching signal; and    -   an operation unit that receives the voltages supplied to said        first to seventh intermediate terminals, and outputs to an        output terminal of said output circuit a voltage obtained by        performing a predetermined operation on the voltages supplied to        said first to seventh intermediate terminals.

In the output circuit according to the present invention, the connectionswitch performs switching between a first connection state and a secondconnection state responsive to the connection switching signal;

-   -   in the first connection state, the connection switch outputs the        first voltage to the first intermediate terminal, outputs the        second voltage to the second and third intermediate terminals,        and outputs the third voltage to the fourth through seventh        intermediate terminals; and    -   in the second connection state, the connection switch outputs        the third voltage to the first through fourth intermediate        terminals, outputs the second voltage to the fifth and sixth        intermediate terminals, and outputs the first voltage to the        seventh intermediate terminal. The operation unit outputs to the        output terminal an average voltage of the voltages supplied to        the first to seventh intermediate terminals.

In the output circuit according to the present invention, the connectionswitch includes:

-   -   a first switch connected between a first terminal with the first        voltage applied thereto and said first intermediate terminal;    -   a second switch connected between a second terminal with the        second voltage applied thereto and said second intermediate        terminal;    -   a third switch connected between the second terminal and said        third intermediate terminal;    -   a fourth switch connected between said third terminal with the        third voltage applied thereto, and said fifth intermediate        terminal;    -   a fifth switch connected between said third terminal and said        sixth intermediate terminal;    -   a sixth switch connected between said third terminal and said        seventh intermediate terminal;    -   said first to sixth switches being on/off controlled by the        connection switching signal;    -   a seventh switch connected between said first terminal and said        seventh intermediate terminal;    -   an eighth switch connected between said second terminal and said        fifth intermediate terminal;    -   a ninth switch connected between said second terminal and said        sixth intermediate terminal;    -   a tenth switch connected between said third terminal and said        first intermediate terminal;    -   an eleventh switch connected between said third terminal and        said second intermediate terminal; and    -   a twelfth switch connected between said third terminal and said        third intermediate terminal;    -   said seventh to twelfth switches being on/off controlled by a        complementary signal of the connection switching signal;    -   said third terminal being connected to said fourth intermediate        terminal, and the third voltage being output to said fourth        intermediate terminal irrespective of a state of the connection        switching signal.

In the output circuit according to the present invention, the operationunit comprises a differential amplifier and a polarity switch.

The differential amplifier includes:

-   -   first to third differential pairs;    -   first to third current sources that supply currents to said        first to third differential pairs, respectively;    -   a load circuit connected in common to output pairs of said first        to third differential pairs;    -   first outputs of output pairs of said first to third        differential pairs being connected in common to a first        connection node; and second outputs of output pairs of said        first to third differential pairs being connected in common to a        second connection node;    -   an amplification stage having an output terminal thereof        connected to said output terminal of said output circuit; and    -   a switching circuit that connects said first connection node or        said second connection node to an input terminal of said        amplification stage responsive to a predetermined control        signal.        The polarity switch performs switching between a first        connection state and a second connection state responsive to the        control signal; wherein in the first connection state, said        first to third intermediate terminals of said connection switch        are connected to first inputs of respective input pairs of said        first to third differential pairs, respectively, and said output        terminal of said differential amplifier being connected to        second inputs of said respective input pairs of said first to        third differential pairs, and    -   in the second connection state, said output terminal of said        differential amplifier is connected to the first inputs of said        respective input pairs of said first to third differential        pairs, and said first to third intermediate terminals of said        connection switch are connected to the second inputs of said        respective input pairs of said first to third differential        pairs, respectively.

In the present invention, in the differential amplifier, sizes ofdevices forming the first to third differential pairs are set to beequal to one another; and

-   -   current values of the first to third current sources are set to        be equal to one another.

The operation unit according to the present invention comprises adifferential amplifier and a polarity switch.

The differential amplifier includes:

-   -   first to seventh differential pairs;    -   first to seventh current sources that supply currents to said        first to seventh differential pairs, respectively;    -   a load circuit connected in common to output pairs of said first        to seventh differential pairs;    -   first outputs of output pairs of said first to seventh        differential pairs being connected in common to a first        connection node; and second outputs of output pairs of said        first to seventh differential pairs being connected in common to        a second connection node;    -   an amplification stage having an output terminal thereof        connected to said output terminal of said output circuit; and    -   a switching circuit that connects said first connection node or        said second connection node to an input terminal of said        amplification stage responsive to a predetermined control        signal.        The polarity switch performs switching between a first        connection state and a second connection state responsive to the        control signal; wherein    -   in the first connection state, said first to seventh        intermediate terminals of said connection switch are connected        to first inputs of respective input pairs of said first to        seventh differential pairs, respectively, and said output        terminal of said differential amplifier is connected to second        inputs of said respective input pairs of said first to seventh        differential pairs; and    -   in the second connection state, said output terminal of said        differential amplifier is connected to the first inputs of said        respective input pairs of said first to seventh differential        pairs, and said first to seventh intermediate terminals of said        connection switch are connected to the second inputs of said        respective input pairs of said first to seventh differential        pairs, respectively.

In the present invention, in the differential amplifier, sizes ofdevices forming the first to seventh differential pairs are set to beequal to one another; and

-   -   current values of the first to seventh current sources are set        to be equal to one another.

In the present invention, the connection switching signal that controlsthe connection switch and the control signal that controls the switchingdevice are identical.

In the connection switch in the present invention, a plurality of theswitches controlled by the same connection switching signal and havingthe same input voltage applied thereto are omitted, except one of theplurality of the switches.

In the present invention, the connection switch performs switchingbetween the first connection state and the second connection state at apredetermined time interval, responsive to the connection switchingsignal; and

-   -   the output circuit outputs the voltage obtained by performing        time averaging of the output voltage of the operation unit in        the first connection state and the output voltage of the        operation unit in the second connection state.

An output circuit according to another aspect of the present inventionincludes:

-   -   a connection switch that includes first through Mth terminals        for receiving first through Mth voltages (V1, V2, . . . , VM),        respectively, and first to (2^(M)−1)th intermediate terminals,        and that selects and supplies:    -   the voltage Vi (where i is an integer greater than or equal to 1        and less than or equal to M) to 2^((i−1)) of the first to        (2^(M)−1)th intermediate terminals, that is,    -   the voltage V1 to one of the first to (2^(M)−1)th intermediate        terminals,    -   the voltage V2 to two of the first to (2^(M)−1)th intermediate        terminals,    -   the voltage V3 to four of the first to (2^(M)−1)th intermediate        terminals, and    -   the voltage VM to 2^(M−1) of the first to (2^(M)−1)th        intermediate terminals, wherein said connection switch switches        assignment of the first through Mth voltages to said first to        (2^(M)−1)th intermediate terminals; and    -   an operation unit that receives the voltages supplied to the        first to (2^(M)−1)th intermediate terminals, and outputs to an        output terminal of said output circuit an average voltage of the        voltages supplied to the first to (2^(M)−1)th intermediate        terminals.

In the present invention, said operation unit comprises a differentialamplifier and a polarity switch.

The differential amplifier includes:

-   -   first to (2^(M)−1)th differential pairs;    -   said output terminal;    -   first to (2^(M)−1)th current sources that supply currents to        said first to (2^(M)−1)th differential pairs, respectively;    -   a load circuit connected in common to output pairs of said first        to (2^(M)−1)th differential pairs;    -   first outputs of output pairs of said first to (2^(M)−1)th        differential pairs being connected in common to a first        connection node; and second outputs of output pairs of said        first to (2^(M)−1)th differential pairs being connected in        common to a second connection node;    -   an amplification stage with an output terminal thereof connected        to said output terminal; and    -   a switching circuit that connects a first connection node or a        second connection node to an input terminal of said        amplification stage responsive to a predetermined control        signal.        The polarity switch performs switching between a first        connection state and a second connection state, wherein    -   in the first connection state, said first to (2^(M)−1)th        intermediate terminals of said connection switch are connected        to first inputs of respective input pairs of said first to        (2^(M)−1)th differential pairs, respectively, and said output        terminal of said differential amplifier is connected to second        inputs of said respective input pairs of said first to        (2^(M)−1)th differential pairs; and    -   in the second connection state, said output terminal of said        differential amplifier being connected to the first inputs of        said respective input pairs of said first to (2^(M)−1)th        differential pairs and said first to (2^(M)−1)th intermediate        terminals of said connection switch being connected to the        second inputs of said respective input pairs of said first to        (2^(M)−1)th differential pairs, respectively.

In the present invention, the connection switch performs switchingbetween a first connection state and a second connection stateresponsive to the connection switching signal;

-   -   in the first connection state, the voltage VM is assigned to        2^(M−1) of the first to (2^(M)−1) intermediate terminals, the        voltage V1 is assigned to one of a remainder of the first to        (2^(M)−1) intermediate terminals, the voltage V2 is assigned to        two of the remainder of the first to (2^(M)−1) intermediate        terminals, the voltage V3 is assigned to four of the remainder        of the first to (2^(M)−1) intermediate terminals, the voltage        V(M−1) is assigned to 2^((M−2)) of the remainder of the first to        (2^(M)−1) intermediate terminals, wherein the voltage Vi (where        i is an integer greater than or equal to 1 and less than or        equal to M−1) is assigned to 2^((i−1)) of the remainder of the        first to (2^(M−)1) intermediate terminals; and    -   in the second connection state, the voltage VM remains to be        assigned to one of the (2^(M)−1) intermediate terminals with the        voltage VM assigned thereto in the first connection state, the        voltage V1 is assigned to one of remaining (2^((M−1))−1)        intermediate terminals, the voltage V2 is assigned to two of the        remaining (2^((M−1))−1) intermediate terminals, the voltage V3        is assigned to four of the remaining (2^((M−1))−1) intermediate        terminals, the voltage V(M−1) is assigned to 2^((M−2)) of the        remaining (2^((M−1))−1) intermediate terminals, wherein the        voltage Vi (where i is an integer greater than or equal to 1 and        less than or equal to M−1) is assigned to 2^((i−1)) of the        remainder of the first to (2^(M)−1) intermediate terminals; and        the voltage VM is assigned to all of (2^((M−1))−1) intermediate        terminals to which the voltages V1 to V(M−1) are assigned in the        first connection state.

A data driver according to the present invention that drives data linesbased on an input digital data signal, includes the output circuit.

The data driver according to the present invention includes:

-   -   a plurality of the output circuits that drive the data lines,        respectively; and    -   a connection switching signal that controls the respective        connection switches of said output circuits;    -   wherein the output circuits are divided into two groups, the two        groups of the output circuits being controlled by said        connection switching signal such that when one group of the        output circuits are in the first connection state, the other        group of the output circuits are in the second connection state,        and that when the one group of the output circuits are in the        second connection state, the other group of the output circuits        are in the first connection state.

A display device according to the present invention includes:

-   -   a data driver including the output circuit; and    -   a display panel;    -   based on an output signal of the data driver, a data line of the        display panel being driven.

A display device according to the present invention includes:

-   -   a plurality of data lines arrayed in parallel to one another in        one direction;    -   a plurality of scan lines arrayed in parallel to one another in        a direction orthogonal to the one direction;    -   a plurality of pixel electrodes arranged at intersections        between the data lines and the scan lines, in a matrix form;    -   a plurality of transistors, each having one of a drain and a        source thereof connected to a corresponding one of said pixel        electrodes, and the other of the drain and the source thereof        connected to a corresponding one of said data lines and a gate        thereof connected to a corresponding one of said scan lines,        each of said transistors corresponding to each of said pixel        electrodes;    -   a gate driver that supplies a scan signal to each of said scan        lines; and    -   the data driver that supplies a gray scale signal corresponding        to input data to each of said data lines.

The meritorious effects of the present invention are summarized asfollows.

According to the present invention, mismatching between an invertinginput side and a non-inverting input side of an amplifier having threeor more differential pairs and mismatching among the differential pairsare canceled by switching of the small number of connection states. Areasaving and reduction of mutual voltage variations among outputs can bethereby implemented.

Further, the present invention accomplishes an effect that, by using theoutput circuit described above, a low-cost data driver with a saved areais made possible, and that cost reduction, reduction of the width of aframe, and higher image quality of a display device including the datadriver can also be implemented.

Still other features and advantages of the present invention will becomereadily apparent to those skilled in this art from the followingdetailed description in conjunction with the accompanying drawingswherein examples of the invention are shown and described, simply by wayof illustration of the mode contemplated of carrying out this invention.As will be realized, the invention is capable of other and differentexamples, and its several details are capable of modifications invarious obvious respects, all without departing from the invention.Accordingly, the drawing and description are to be regarded asillustrative in nature, and not as restrictive.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of an output circuit in afirst embodiment of the present invention;

FIG. 2 is a diagram showing a configuration of a digital-to-analogconverter circuit (DAC) in the first embodiment of the presentinvention;

FIG. 3 is a table showing relationships between bit data and outputs ofa selection circuit in the first embodiment of the present invention;

FIG. 4 is a graph showing a relationship between an output voltage ofthe selection circuit for each gray scale and an output voltage of anoperation unit in the first embodiment of the present invention;

FIG. 5 is a diagram showing a configuration example of a decoder in thefirst embodiment of the present invention;

FIG. 6 is a diagram showing a configuration example of the operationunit in the first embodiment of the present invention;

FIGS. 7A, 7B, and 7C are graphs plotting output errors of the operationunit in the first embodiment of the present invention and average outputerrors when connection switching is performed;

FIGS. 8A, 8B, and 8C are graphs plotting DNLs of the operation unit inthe first embodiment of the present invention and average DNLs when theconnection switching is performed;

FIG. 9 is a diagram showing a configuration example of a connectionswitch in the first embodiment of the present invention;

FIG. 10 is a diagram showing a configuration example in which theconnection switch and a polarity switch in the first embodiment of thepresent invention are synthesized;

FIG. 11 is a diagram showing a configuration of an output circuit in asecond embodiment of the present invention;

FIG. 12 is a diagram showing a configuration of a digital-to-analogcircuit (DAC) in the second embodiment of the present invention;

FIG. 13 is a table showing relationships between bit data and outputs ofa selection circuit in the second embodiment of the present invention;

FIG. 14 is a graph showing a relationship between an output voltage ofthe selection circuit for each gray scale and an output voltage of anoperation unit in the second embodiment of the present invention;

FIG. 15 is a diagram showing a configuration example of a decoder in thesecond embodiment of the present invention;

FIG. 16 is a diagram showing a configuration example of the operationunit in the second embodiment of the present invention;

FIGS. 17A, 17B, and 17C are graphs plotting output errors of theoperation unit in the second embodiment of the present invention andaverage output errors when connection switching is performed;

FIGS. 18A, 18B, and 8C are graphs plotting DNLs of the operation unit inthe second embodiment of the present invention and average DNLs when theconnection switching is performed;

FIG. 19 is a diagram showing a configuration example of a connectionswitch in the second embodiment of the present invention;

FIG. 20 is a diagram showing another configuration example of theconnection switch in the second embodiment of the present invention;

FIG. 21 is a diagram showing a configuration example in which theconnection switch and a polarity switch in the second embodiment of thepresent invention are synthesized;

FIG. 22 is a diagram showing another configuration example in which theconnection switch and the polarity switch in the second embodiment ofthe present invention are synthesized;

FIG. 23 is a diagram showing a configuration example when the presentinvention has been applied to a data driver;

FIG. 24 is a diagram showing a configuration example when the datadriver utilizing the present invention has been applied to an activematrix type liquid crystal display device;

FIG. 25 is a diagram showing a configuration of the active matrix typeliquid crystal display device;

FIG. 26 is a diagram showing a conventional data driver;

FIG. 27 is a diagram showing a configuration of a DAC described inPatent Document 1; and

FIG. 28 is a diagram showing a configuration of a differential amplifierdescribed in Patent Document 2.

PREFERRED MODES OF THE INVENTION

The preferred modes of the present invention will be described withreference to the drawings. FIG. 1 is a diagram showing a configurationof an embodiment of the present invention. Referring to FIG. 1, aconnection switch 11 receives a first voltage V1 and a second voltage V2from a terminal Tin1 and a terminal Tin2, respectively. According to aconnection switching signal, a connection state between each of theterminals Tin1 and Tin2 to which the voltages V1 and V2 are input,respectively, and each of intermediate terminals T1, T2, and T3 isswitched. Then, the voltage V1 or V2 is selected and output to each ofthe intermediate terminals T1, T2, and T3, including selection of thesame voltage for a plurality of the intermediate terminals to a terminalTout.

A operation unit 12 outputs a voltage obtained by averaging voltagesV(T1), V(T2), and V(T3) supplied to the intermediate terminals T1, T2,and T3, respectively, to the output terminal Tout.

Then, by switching the connection switching signal by a predeterminedtime, voltages output from the operation unit 12 in the respectiveconnection states are time averaged.

The connection switch 11 can also switch only positions of theintermediate terminals to which the voltages V1 and V2 are assigned,with the ratio of the number of the voltages V1 and V2 assigned to theintermediate terminals T1, T2, and T3, respectively, maintained at theratio of 1:2.

In this case, it is ideal that the same voltage should be output in anyconnection state. However, actually, an output voltage of the operationunit 12 is slightly deviated from an expected value due to manufacturingvariations of transistors.

However, by performing an operation of switching an input state of theoperation unit 12 by the predetermined time, an error is averaged overthe time. The error caused by device variations of the operation unit 12can be thereby effectively reduced.

In the following examples, in particular, configurations, operations,and an error reduction effect in a case where two or three inputvoltages are used will be described in detail.

A first example of the present invention will be described withreference to FIG. 1. The connection switch 11 receives the first voltageV1 and the second voltage V2 from the terminal Tin1 and the terminalTin2, respectively. Responsive to the connection switching signal, aconnection state between each of the terminals Tin1 and Tin2 to whichthe voltages V1 and V2 are applied, respectively, and each of theintermediate terminals T1, T2, and T3 is switched. Then, the connectionswitch 11 selects and outputs the voltage V1 or V2 to each of theintermediate terminals T1, T2, and T3, including selection of the samevoltage for a plurality of the intermediate terminals.

The connection switch 11 performs switching between a first connectionstate and a second connection state responsive to the connectionswitching signal.

In the first connection state, the connection switch 11 outputs thevoltage V1 to the intermediate terminal T1, and outputs the voltage V2to the intermediate terminals T2 and T3. (T1, T2, T3)=(V1, V2, V2)

In the second connection state, the connection switch 11 outputs thevoltage V1 to the intermediate terminal T3, and outputs the voltage V2is output to the intermediate terminals T2 and T1. (T1, T2, T3)=(V2, V2,V1).

The operation unit 12 outputs to the output terminal Tout an averagevoltage of the voltages V(T1), V(T2), and V(T3) supplied to theintermediate terminals T1, T2, and T3, respectively.

One of the voltages V(T1), V(T2), and V(T3) becomes the voltage V1 andtwo of the voltages V(T1), V(T2), and V(T3) become the voltages V2 bythe connection switch 11. An output voltage Vout output to the outputterminal Tout is therefore given by the following expression (1):

$\begin{matrix}{V_{out} = \frac{1 - V_{1} + 2 - V_{2}}{3}} & (1)\end{matrix}$

That is, the voltage obtained by taking weighted average of the voltagesV1 and V2 at a ratio of 1:2 is output.

Further, using this connection switch and this operation unit, a DAC (adigital-to-analog converter) can be formed.

FIG. 2 is a diagram showing a configuration of a 6 bit-DAC in thisexample. A selection circuit 13 selects two voltages inclusive of thetwo voltages that may be identical from among eight reference voltagesaccording to 6-bit data and assigns the two voltages to the terminalsTin1 and Tin2, respectively. Since the connection switch 11 and theoperation unit 12 in FIG. 2 have the same configurations as those inFIG. 1, descriptions of the connection switch 11 and the operation unit12 will be omitted.

Among equally spaced voltages of 64 levels (corresponding to six bits),first, fourth, thirteenth, sixteenth, forty-ninth, fifty-second,sixty-first, and sixty-fourth voltages (these voltage being representedby VG00, VG03, VG12, VG15, VG48, VG51, VG60, and VG63, respectively) areemployed as eight reference voltages. The selection circuit 13 selectsthe voltages V1 and V2 supplied to the terminals Tin1 and Tin2,respectively, according to FIG. 3. That is, linear output voltages of 64levels from 0th gray scale corresponding to 6-bit data (D5, D4, D3, D2,D1, D0)=(0, 0, 0, 0, 0, 0) to 63th gray scale corresponding to (D5, D4,D3, D2, D1, D0)=(1, 1, 1, 1, 1, 1) can be obtained. FIG. 4 is a graphshowing a 64 gray scale-output voltage characteristic.

As an example of the selection circuit 13 in this case, a configurationas shown in FIG. 5 is employed. Three branch stages, each of which isbranched into two signal paths, are provided from the terminal Tin1 toobtain eight branched signal paths. Then, switches are provided in eachof the branch stages, and selection of one of the voltages VG00, VG03,VG12, VG15, VG15, VG48, VG51, VG60, and VG63 is performed. In each ofthe branch stages, a switch for one of complementary signals DOB and D0,D2B and D2, and D4B and D4 connected to a signal at a high level isturned on. Three branch stages, each of which is branched into twosignal paths, are provided from the terminal Tin2, and selection of oneof the voltages VG00, VG03, VG12, VG15, VG48, VG51, VG60, and VG63 isperformed. In each of the branch stages, a switch connected to one ofcomplementary signals D1B and D1, D3B and D3, and D5B and D5 connectedto a signal at the high level is turned on. When (D0, D1, D2, D3, D4,D5)=(0, 0, 0, 0, 0, 0), for example, the same voltage VG00 is selectedfor the terminals Tin1 and Tin2. When (D0, D1, D2, D3, D4, D5=(1, 0, 0,0, 0, 0), the voltages VG03 and VG00 are selected for the terminals Tin1and Tin2, respectively.

With the above described selection circuit 13 and the operation unit 12,8²=64 voltages from among the eight reference voltages can be output tothe output terminal Tout of the operation unit.

FIG. 6 is a diagram showing an example of a configuration of theoperation unit 12. Referring to FIG. 6, this operation unit 12 includesa first differential pair Dif1, a second differential pair Dif2, a thirddifferential pair Dif3, the output terminal Tout, a first current sourceCS1, a second current source CS2, and a third current source CS3 thatsupply currents to the first, second, and third differential pairs,respectively, and a load circuit L1 connected in common to output pairsof the first differential pair Dif1, second differential pair Dif2, andthird differential pair Dif3.

First outputs (drains of transistors M1P, M2P, and M3P) of therespective output pairs of the first differential pair Dif1, seconddifferential pair Dif2, and third differential pair Dif3 are connectedin common to a first connection node N1. Second outputs (drains oftransistors M1M, M2M, and M3M) of the respective output pairs of thefirst differential pair Dif1, second differential pair Dif2, and thirddifferential pair Dif3 are connected in common to a second connectionnode N2.

The first connection node N1 is connected to a drain of a P-channeltransistor PM1 that constitutes the load circuit L1. The secondconnection node N2 is connected to a drain of a P-channel transistor PM2that constitutes the load circuit L1. Gates of the P-channel transistorsPM1 and PM2 are coupled together, and sources of the P-channeltransistors PM1 and PM2 are connected in common to a power supply VDD. Aswitch 122M is connected between a gate of the P-channel transistor PM1and the drain of the P-channel transistor PM1. A switch 122P isconnected between a gate of the P-channel transistor PM2 and the drainof the P-channel transistor PM2. The switch 122P is turned on when acontrol signal S is high, and the switch 122M is turned on when thecontrol signal S is low.

The operation unit 12 includes switching circuits 123P and 123M thatperform switching between connection of the first connection node N1 toan input terminal of an amplification stage A1 and connection of thesecond connection node N2 to the input terminal of the amplificationstage A1, responsive to the control signal S and a control signal SB.

A polarity switch (a plus/minus switch) 121 performs interchange betweennon-inverting inputs (+) and inverting inputs (−) of the differentialpairs. The polarity switch 121 includes switches that receive signals atthe internal terminals T1, T2, and T3, respectively, and an output ofthe amplification stage A1, and switch connections to terminals T1P,T2P, T3P, T1M, T2M, and T3M, respectively. The terminals T1P, T2P, andT3P are connected to first inputs (gates of the transistors M1P, M2P,and M3P) of the first differential pair Dif1, second differential pairDif2, and third pair Dif3, respectively. The terminals T1M, T2M, and T3Mare connected to second inputs (gates of the transistors M1M, M2M, andM3M) of the first differential pair Dif1, second differential pair Dif2,and third differential pair Dif3, respectively.

When the control signal S is high, a switch 1211 is turned on, and aswitch 1212 is turned off. The intermediate terminals T1, T2, and T3 areconnected to the terminals T1P, T2P, and T3P, respectively, and theoutput terminal Tout of the amplification stage A1 is connected to theterminals T1M, T2M, and T3M, respectively.

The intermediate terminals T1, T2, and T3 are connected to the firstoutputs (gates of the transistors M1P, M2P, M3P) of the firstdifferential pair Dif1, second differential pair Dif2, and thirddifferential pair Dif3, respectively. The output terminal Tout isconnected to the second inputs (gates of the transistors M1M, M2M, andM3M).

When the control signal S is low, the switch 1212 is turned on, and theswitch 1211 is turned off. The intermediate terminals T1, T2, and T3 areconnected to the terminals T1M, T2M, and T3M, respectively, and theoutput terminal Tout of the amplification stage A1 is connected to theterminals T1P, T2P, and T3P, respectively. The intermediate terminalsT1, T2, and T3 are connected to the second inputs (gates of thetransistors M1M, M2M, M3M) of the first differential pair Dif1, seconddifferential pair Dif2, and third differential pair Dif3, respectively.The output terminal Tout is connected to the first inputs (gates of thetransistors M1P, M2P, and M3P).

When sizes of the transistors M1P to M3P and the transistors M1M to M3Mthat constitute the differential pairs Dif1, Dif2, and Dif3 are equal toone another, when current values of the current sources CS1 to CS3 areset to be equal to one another, and when voltages (of approximately0.2V) input to the terminals T1 to T3, respectively, are close to oneanother, an output voltage becomes a value given by Expression (1).

Further, using the polarity switch 121, and the switches 122P, 122M,123P, and 123M, non-inverting inputs (+) and inverting inputs (−) of adifferential amplifier circuit can be interchanged.

That is, when the switches 1211 of the polarity switch 121 are turned onand the switches 1212 of the polarity switch 121 are turned off, andwhen the switches 122P and 123P are turned on and the switches 122M and123M are turned off, the gates of the transistors M1P, M2P, and M3P ofthe differential pairs Dif1 to Dif3 become the non-inverting inputs (+)and the gates of the transistors M1M, M2M, and M3M of the differentialpairs Dif1 to Dif3 become the inverting inputs (−).

Conversely, when the switches 1212 of the polarity switch 121 are turnedon and the switches 1211 of the polarity switch 121 are turned off, andwhen the switches 122M and 123M are turned on and the switches 122P and123P are turned off, the gates of the transistors M1M, M2M, and M3M ofthe differential pairs Dif1 to Dif3 become the non-inverting inputs (+)and the gates of the transistors M1P, M2P, and M3P of the differentialpairs Dif1 to Dif3 become the inverting inputs (−).

When the operation unit 12 is an ideal operation unit, the voltageexpressed by Expression (1) is output to the output terminal Tout,irrespective of a connecting state.

Actually, however, an operation error or an offset caused bymanufacturing variations (mismatching) among transistors in theoperation unit 12 often occurs. When the operation unit 12 is employedfor the DAC as shown in FIG. 2, and when the variations are large, inparticular, gray scale inversion or a gray scale jump may also occur.

In order to exhibit an effect achieved by the present invention, maximumand minimum possible values of a time averaging of offset voltages wereshown in FIGS. 7A-7C, and maximum and minimum possible values of a timeaveraging of adjacent gray scale voltages (DNL: DifferentialNon-Linearity) were shown in FIGS. 8A-8C. These operations wereperformed under a condition in which transistor sizes of thedifferential pairs Dif1 to Dif3, current sources CS1 to CS3, and loadcircuit L1 of the operation unit 12 in FIG. 6 were varied randomly, andby application of the present invention, the first connection state andthe second connection state were temporally switched.

DNL is a value expressing the linearity of the DAC. DNL is thedifference between changes of a DAC's actual analog output relative to asingle step change (1LSB). The more DNL is close to zero, it shows thatthe linearity is more satisfactory (and is close to an ideal straightline). When the DNL exceeds one, it shows that the gray scale jumpoccurs between adjacent gray scales. When the DNL becomes equal to orless than −1, it shows that gray scale inversion occurs between theadjacent gray scales.

As an object of comparison for showing the effect of the presentinvention, the time averaging when no interchange among the differentialpairs was made and only the non-inverting inputs (+) and the invertinginputs (−) of the differential pairs were switched was also shown ineach of FIGS. 7A-7C and 8A-8C.

Referring to FIGS. 7A-7C and 8A-8C, it can be seen that the offsetvoltage and the DNL have been improved more than in a state A (aconnection state 1) where no switching was performed, and that the DNLbetween 31th gray scale and 32th gray scale in particular has beengreatly improved.

It can be seen that in an example of the comparison, an offset voltagehas been improved by switching between the non-inverting inputs and theinverting inputs of the differential pairs (refer to FIG. 7C), but noimprovement has been made on the DNL (refer to FIG. 8C).

This has shown that, by applying the present invention, the DNL as wellas the offset voltage can be improved.

Next, a configuration of the connection switch 11 in this example willbe described.

In the first connection state, the connection switch 11 outputs thevoltage V1 to the intermediate terminal T1 and outputs the voltage V2 tothe intermediate terminals T2 and T3, among the intermediate terminalsT1 to T3.

In the second connection state, the connection switch 11 outputs thevoltage V1 to the intermediate terminal T3 and outputs the voltage V2 tothe intermediate terminals T2 and T1, among the intermediate terminalsT1 to T3.

Accordingly, more specifically, the terminal Tin1 to which the voltageV1 is input and the intermediate terminal T1 are connected by a switchcontrolled by a connection switching signal (CP), and the terminal Tin2to which the voltage V2 is input and the intermediate terminal T3 areconnected by a switch controlled by the connection switching signal(CP).

The terminal Tin1 to which the voltage V1 is input and the intermediateterminal T3 are connected by a switch controlled by a complementarysignal (CPB) of the connection switching signal, and the terminal Tin2to which the voltage V2 is input and the intermediate terminal T1 areconnected by a switch controlled by the complementary signal (CPB) ofthe connection switching signal.

The intermediate terminal T2 should have a configuration in which thevoltage V2 is output therefrom irrespective of a state of the connectionswitching signal. The configuration that meets the specification asdescribed above becomes the one as shown in FIG. 9, for example. Aswitch SW11 is provided between the terminal Tin1 and the terminal T1. Aswitch SW13 is provided between the terminal Tin1 and the terminal T3. Aswitch SW21 is provided between the terminal Tin2 and the terminal T1. Aswitch 23 is provided between the terminal Tin2 and the terminal T3.Then, the terminal Tin2 is directly coupled to the terminal T2. Theswitches SW11 and SW23 constitute switches 1101. Turning on and off ofthe switches SW11 and SW23 is controlled by the connection switchingsignal CP. The switches SW21 and SW13 constitute switches 1102. Turningon and off of the switches SW21 and SW13 is controlled by the connectionswitching signal CPB (complementary signal of the signal CP).

Referring to FIG. 9, in the first connection state, the switches 1101are turned on, and the switches 1102 are turned off. The voltage V1 isthereby output to the intermediate terminal T1, and the voltage V2 isthereby output to the intermediate terminals T2 and T3. In the secondconnection state, the switches 1102 are turned on, and the switches 1101are turned off. The voltage V1 is thereby output to the intermediateterminal T3, and the voltage V2 is output to the intermediate terminalsT2 and T1. Though the voltage V2 and the intermediate terminal T2 areshort-circuited, the voltage V2 should be output to the intermediateterminal T2, irrespective to the connection state.

The switches 1101 and the switches 1102 in FIG. 9 can be formed of MOStransistors. That is, one of a source and a drain of each MOS transistorshould be connected to the terminal (Tin1 or Tin2) to which the voltageV1 (or V2) is input, the other of the source and the drain of each MOStransistor should be connected to the intermediate terminal (T1 or T2),and the connection switching signal (CP) or the complementary signal(CPB) of the connection switching signal should be input to a gate ofeach MOS transistor. When the switches are formed of N-channeltransistors, the switches are turned on when the connection switchingsignal (CP) is high. When the connection switching signal (CP) is low,the switches are turned off. Thus, in the first connection state, theconnection switching signal (CP) should be set to high, while in thesecond connection state, the connection switching signal (CP) should beset to low.

When the switches are formed of P-channel transistors, an on/off logicbecomes the reverse of that of the N-channel transistors. Thus, in thefirst connection state, the connection switching signal (CP) should beset to low. In the second connection state, the connection switchingsignal (CP) should be set to high. Further, each switch may be formed ofa transfer gate in which an N-channel transistor is combined with aP-channel transistor.

Further, the signal that controls the polarity switch 121 and the signalthat controls the connection switch 11 can be made the same. In thiscase, the polarity switch 121 and the connection switch 11 may becombined into one connection switch.

FIG. 10 shows an example of a configuration in which the polarity switch121 and the connection switch 11 are combined into one connection switch11B in this example.

Referring to FIG. 10, in the first connection state, switches 1111 areturned on, and switches 1112 are turned off. The voltage V1 is therebyoutput to the terminal T1P, the voltage V2 is thereby output to theterminals T2P and T3P, and a voltage Tout is output to the terminalsT1M, T2M, and T3M. In the second connection state, the switches 1112 areturned on, and the switches 1111 are turned off. The voltage V1 isthereby output to the terminal T3M, the voltage V2 is thereby output tothe terminals T2M and TIM, and the voltage Tout is output to theterminals T1P, T2P, and T3P. As described above, the polarity switch 121and the connection switch are configured to be combined, therebyallowing reduction of the total number of the switches.

FIG. 11 is a diagram showing a configuration of a second example of thepresent invention. First, second, and third voltages (V1, V2, V3) aresupplied to a connection switch 11, and connection states betweenrespective terminals to which the voltages V1, V2, and V3 are input andrespective intermediate terminals T1 to T7 are switched responsive to aconnection switching signal. The connection switch 11 performs selectionamong the voltages V1, V2, and V3 including selection of the voltagesthat may be identical and outputs the selected voltages to the terminalsT1 to T7, respectively.

Alternatively, the connection switch 11 performs switching between afirst connection state and a second connection state responsive to theconnection switching signal, and outputs the voltage V1 to the terminalT1, outputs the voltage V2 to the terminals T2 and T3, and outputs thevoltage V3 to the terminals T4, T5, T6, and T7, among the terminals T1to T7, in the first connection state.

In the second connection state, the connection switch 11 outputs thevoltage V1 to the terminal T7, outputs the voltage V2 to the terminalsT6 and T5, and outputs the voltage V3 to the terminals T4, T3, T2, andT1, among the terminals T1 to T7.

A operation unit 12 outputs an average voltage of seven voltages V(T1)to V(T7) supplied to the terminals T1 to T7, and outputs the averagevoltage to an output terminal Tout.

One of the voltages V(T1) to V(T7) becomes the voltage V1, two of thevoltages V(T1) to V(T7) become the voltages V2, and four of the voltagesV(T1) to V(T7) become the voltages V3, by the connection switch 11. Anoutput voltage Vout is as follows:

Vout=(1·V ₁+2·V ₂+4·V ₃)/7  (2)

That is, the voltage obtained by taking weighted average of the voltagesV1, V2, and V3 at a ratio of 1:2:4 is output.

Further, using this connection switch 11 and the operation unit 12, aDAC (digital-to-analog converter) can be composed.

FIG. 12 is a diagram showing a concept about a configuration of a 6-BitDAC in this example. A selection circuit 13 has a function of selectingthree voltages that may be identical from among four reference voltagesaccording to 6-bit data, and assigning the selected three voltages toterminals Tin1 to Tin3.

Among voltages of 64 levels (corresponding to six bits) equally spaced,first, eighth, fifty-seventh, sixty-fourth, voltages (these voltagesbeing represented by voltages VG00, VG07, VG56, and VG63, respectively)are set to the four reference voltages. Then, when the selection circuit13 selects the voltages V1 and V3 supplied to the terminals Tin1 andTin3, respectively according to a table shown in FIG. 13, linear outputvoltages of the 64 levels from 0th gray scale corresponding to 6-bitdata (D5, D4, D3, D2, D1, D0)=(0, 0, 0, 0, 0, 0) to 63th gray scalecorresponding to 6-bit data (D5, D4, D3, D2, D1, D0)=(1, 1, 1, 1, 1, 1)can be obtained (as shown in FIG. 14). FIG. 13 shows a list ofrespective bit data of gray scales from 0th gray scale to 63th grayscale and the outputs V1, V2, and V3 of the selection circuit.

As an example, the selection circuit 13 is configured as shown in FIG.15. One of the voltages VG00, VG07, VG56, and VG63 is output to theterminal Tin1 via switches that are turned on according to the bit dataD0 and D3 and complementary signals of the bit data D0 and D3. One ofthe voltages VG00, VG07, VG56, and VG63 is output to the terminal Tin2according to the bit data D1 and D4 and complementary signals of the bitdata D1 and D4. One of the voltages VG00, VG07, VG56, and VG63 is outputto the terminal Tin3 according to the bit data D2 and D5 andcomplementary signals of the bit data D2 and D5. By using the selectioncircuit 13 and the operation unit 12, 4³=64 voltages from among the fourreference voltages can be output to the output terminal Tout of theoperation unit (refer to FIG. 14).

FIG. 16 is a diagram showing a configuration of the operation unit 12 inthis example. Referring to FIG. 16, the operation unit 12 includes firstto seventh differential pairs Dif1 to Dif7, the output terminal Tout,first to seventh current sources CS1 to CS7 that supply currents to thefirst to seventh differential pairs Dif1 to Dif7, respectively, and aload circuit L1 connected in common to output pairs of the first toseventh differential pairs Dif1 to Dif7.

First outputs of the respective output pairs of the first to seventhdifferential pairs Dif1 to Dif7 are connected in common to a firstconnection node N1. Second outputs of the first to seventh differentialpairs Dif1 to Dif7 are connected in common to a second connection nodeN2.

The first connection node N1 is connected to a drain of a P-channeltransistor PM1 that forms the load circuit 11, and the second connectionnode N2 is connected to a drain of a P-channel transistor PM2 that formsthe load circuit L1. Gates of the P-channel transistors PM1 and PM2 areconnected, and sources of the P-channel transistors PM1 and PM2 areconnected to a power supply. A switch 122M is connected between thedrain of the P-channel transistor PM1 and a gate of the P-channeltransistor PM1. A switch 122P is connected between the drain of theP-channel transistor PM2 and a gate of the P-channel transistor PM2. Theswitch 122P is turned on when a control signal S is high. The switch122M is turned on when the control signal S is low.

The operation unit 12 includes a switching circuit 123P and a switchingcircuit 123M that perform switching between connection of the firstconnection node N1 to an input terminal of an amplification stage A1 andconnection of the second connection node N2 to the input terminal of theamplification stage A1.

A polarity switch 121 switches connection of the intermediate terminalsT1 to T7 of the connection switch 11 to first inputs of the first toseventh differential pairs (gates of transistors M1P, M2P, . . . andM7P) and connection of the output terminal Tout of a differentialamplifier to second inputs of the first to seventh differential pairs(gates of transistors M1M, M2M, . . . , and M7M), or connection of theoutput terminal Tout of the differential amplifier to the first inputsof the first to seventh differential pairs (gates of the transistorsM1P, M2P, . . . , M7P) and connection of the intermediate terminals T1to T7 of the connection switch 11 to the second inputs of the first toseventh differential pairs (gates of the transistors M1M, M2M, . . . ,M7M), responsive to the control signal S and a complementary signal SBof the control signal S.

The configuration in FIG. 16 is the configuration in which the number ofthe differential pairs has been increased from three to seven in theoperation unit 12 shown in FIG. 6.

When each size of the transistors M1P to M7P that constitute thedifferential pairs Dif1 to Dif7 is mutually equal to each size of thetransistors M1M to M7M that constitute the differential pairs Dif1 toDif7 in this operation unit 12 (differential amplifier), when respectivecurrent values of the current sources CS1 to CS7 are set to be equal toeach other, and when voltages (of approximately 0.2V) supplied to theterminals T1 to T7, respectively, are close to one another, an outputvoltage becomes a value given by Expression (2).

Further, in the configuration in FIG. 16, using the polarity switch 121,and the switches 122P, 122M, 123P, and 123M, non-inverting inputs (+)and inverting inputs (−) of the differential pairs can be interchanged,as in the operation unit 12 in FIG. 6. Since an operation ofinterchanging the non-inverting inputs (+) and the inverting inputs (−)of the differential pairs is the same as that of FIG. 6, a descriptionabout the operation will be omitted.

As described in the first example, an operation error or an offset oftenoccurs due to manufacturing variations (mismatching) among thetransistors inside the operation unit 12.

Then, in order to exhibit an effect by the present invention, maximumand minimum possible values of a time averaging of offset voltages whentransistor sizes of the differential pairs Dif1 to Dif7 of the operationunit 12 in FIG. 16, current sources CS1 to CS7, and load circuit L1 werevaried randomly, and by applying the present invention, switchingbetween the first connection state and the second connection state wastemporally made, for output, were shown in FIGS. 17A-17C. Then, maximumand minimum possible values of a time averaging of adjacent gray scalevoltages (DNL) were shown in FIGS. 18A-18C.

As an object of comparison for showing the effect of the presentinvention, the time averaging when no interchange among the differentialpairs was made and only the non-inverting inputs (+) and the invertinginputs (−) of the differential pairs were switched was also shown ineach of FIGS. 17C and 18C.

Referring to FIGS. 17A-17C and 18A-18C, it can be seen that the offsetvoltage and the DNL have been improved more than in the state(connection state 1)(A) where no switching was performed, and that theDNL between 31th gray scale and 32th gray scale in particular has beengreatly improved.

It can be seen that in the example of the comparison in each of FIGS.17C and 18C, the offset voltage has been improved by switching betweenthe non-inverting inputs and inverting inputs of the differential pairs,but no improvement has been made on the DNL.

This has shown that, by applying the present invention, the DNL as wellas the offset voltage can be improved.

Next, a specific configuration of the connection switch 11 in thisexample will be described.

In the first connection state, the connection switch 11 outputs thevoltage V1 to the terminal T1, outputs the voltage V2 to the terminalsT2 and T3, and outputs the voltage V3 to the terminals T4, T5, T6, andT7, among the intermediate terminals T1 to T7. In the second connectionstate, the connection switch 11 outputs the voltage V1 to the terminalT7, outputs the voltage V2 to the terminals T6 and T5, and outputs thevoltage V3 to the terminals T4, T3, T2, and T1, among the intermediateterminals T1 to T7.

Accordingly, the terminal Tin1 to which the voltage V1 is input and theterminal T1, the terminal Tin2 to which the voltage V2 is input and eachof the terminals T2 and T3, and the terminal Tin3 to which the voltageV3 is input and each of the terminals T5 to T7 are connected by switchescontrolled by the connection switching signal (CP), respectively. Theterminal Tin1 to which the voltage V1 is input and the terminal T7, theterminal Tin2 to which the voltage V2 is input and each of the terminalsT5 and T6, the terminal Tin3 to which the voltage V3 is input and eachof the terminals T1 to T3 are connected by switches controlled by thecomplementary signal of the connection switching signal, respectively.The voltage V3 is output to the terminal T4 irrespective of theconnection switching signal.

The configuration of the connection switch 11 as described above becomesthe one as shown in FIG. 19, for example. Referring to FIG. 19, in thefirst connection state, switches 1103 composed of switches SW11, SW22,SW23, SW35, SW36, and SW37 are turned on, and switches 1104 composed ofswitches SW31, SW32, SW33, SW25, SW26, and SW17 are turned off. As aresult, the voltage V1 is thereby output to the terminal T1, the voltageV2 is thereby output to the terminals T2 and T3, and the voltage V3 isoutput to the terminals T4, T5, T6, and T7. Meanwhile, a switch Swabindicates the switch connected between a terminal Tina (in which a isone of 1, 2, and 3) and a terminal Tb (in which b is one of 1 to 7).

In the second connection state, the switches 1104 are turned on, and theswitches 1103 are turned off. As a result, the voltage V1 is therebyoutput to the terminal T7, the voltage V2 is output to the terminals 6and 5, and the voltage V3 is output to the terminals T4, T3, T2, and T1.

Though the voltage V3 and the terminal T4 are short-circuited, thevoltage V3 should be output irrespective of the connection state.

In the connection switch 11 shown in FIG. 19, a plurality of switchesconnected to intermediate terminals to which the same voltage is outputmay be combined, irrespective of the connection state. For example, inthe first connection state, the voltage V2 is output to both of theintermediate terminals T2 and T3. In the second connection state, thevoltage V3 is output to both of the intermediate terminals T2 and T3.

Accordingly, even in both of the connection states, the same voltage isoutput to the intermediate terminals T2 and T3. Thus, as for theintermediate terminals T2 and T3 and the switches 1103 on/off controlledby the connection control signal CP, switches to which the voltage V2 isapplied may be combined into one switch.

A configuration in which the switches are combined as described above inthe connection switch 11 in FIG. 19 is shown in FIG. 20. Referring toFIG. 20, the intermediate terminals T2 and T3 in FIG. 19 are combinedinto one terminal (with common switches) and the intermediate terminalsT5 and T6 in FIG. 19 are combined into one terminal (with commonswitches). In the case of the configuration shown in FIG. 20, the numberof the switches can be reduced more than in the configuration in FIG.19. Meanwhile, each switch in FIGS. 19 and 20 can be formed of anN-channel transistor or a P-channel transistor, as in the first example.

Further, when the same signal controls the polarity switch 121 and theconnection switch 11, as in the first example, the polarity switch 121and the connection switch 11 may be combined into one connection switch.

FIG. 21 shows one connection switch 11B obtained by combining thepolarity switch 121 and the connection switch 11 in this example, andthe number of the switches is reduced. When the connection switchingsignal CP is high, switches 1113 are turned on, and switches 1114 areturned off. Then, the terminal Tin1 is connected to a terminal T1P, theterminal T1 n 2 is connected to terminals T2P and T3P, and the terminalTin3 is connected to terminals T4P, TSP, T6P, and T7P. The outputterminal Tout is connected to terminals T1M to T7M. When the connectionswitching signal CP is low (when the signal CPB is high), the switches1114 are turned on, and the switches 1113 are turned off. Then, theterminal Tin1 is connected to the terminal T7M, the terminal Tin2 isconnected to the terminals T5M and T6M, and the terminal Tin3 isconnected to the terminals T1M, T2M, T3M, and T4M. The output terminalTout is connected to the terminals T1P to T7P.

In the same manner in which the configuration in FIG. 19 is modifiedinto the configuration in FIG. 20, switches having the same input andoutput in the first and second connection states can be combined intoone switch in the configuration in FIG. 21 as well. The configuration inFIG. 21 can be modified into a configuration in FIG. 22, for example.The terminals T2P and T3P are combined into one terminal (with commonswitches). The terminals T5P and T6P are combined into one terminal(with common switches). The terminals T2M and T3M are combined into oneterminal (with common switches). Then, the terminals T5M and T6M arecombined into one terminal (with common switches).

The above description showed examples where the present invention hasbeen applied to two examples when the three differential pairs were usedand the seven differential pairs were used. When examples in FIGS. 2 and12 are expanded, the following description can be generally made.

That is, a DAC formed of a selection circuit that selects M voltages V1to VM inclusive of voltages that may be identical from among 2Kreference voltages, a connection switch including M terminals to whichthe voltages V1 to VM are input, respectively, and (2^(M)−1)intermediate terminals T1 to T(2^(M)−1), and an operation unit thatoutputs to the output terminal Tout an average voltage of voltages V(T1)to V(T(2^(M)−1)) given to the (2^(M)−1) intermediate terminals T1 toT(2^(M)−1), respectively, can be configured. Among the intermediateterminals T1 to T(2^(M)−1), the connection switch outputs the voltage V1to one of the intermediate terminals, outputs the voltage V2 to two ofthe intermediate terminals, outputs the voltage V3 to four of theintermediate terminals, outputs a voltage Vi (i being an integer notless than one and not more than M) to 2(^(i−1)) of the intermediateterminals, and outputs the voltage VM to 2(^(M−1)) of the intermediateterminals.

In this DAC, the reference voltages are set to

-   -   (1+(2^(M)−1)Σ_(i=1) ^(K)(α_(i)×2^((i−1)M))th voltages (where        α_(i) to α_(K) assume one of values of 0 and 1) among voltages        of 2^(KM) levels equally spaced. Equally spaced 2^(KM) output        voltages are thereby obtained.

In both of the examples described in the first and second examples, thefollowing equation holds:

-   -   the number of the intermediate terminals (such as the terminal        T1)=(square of the number of input voltages (such as the voltage        V1)−1

Further, it can be seen that one of the voltages V1 to V3 is alwaysassigned to (N+1)/2 intermediate terminals of all the (N) intermediateterminals. It can be seen that when the voltage assigned to the (N+1)/2intermediate terminals in the first connection state is represented byVx, switching is performed in the second connection state so that thevoltage assigned to one of the (N+1)/2 intermediate terminals to whichthe voltage Vx has been assigned in the first connection state remainsto be the voltage Vx, the voltage other than the voltage Vx is assignedto remaining (N−1)/2 intermediate terminals, and the voltage Vx isassigned to (N−1)/2 intermediate terminals to which the voltage otherthan the voltage Vx has been assigned in the first connection state.

The voltage corresponding to the above-mentioned voltage Vx is V3 in thesecond example of the present invention, and the voltage V3 is assignedto (7+1)/2=4 intermediate terminals among the seven intermediateterminals T1 to T7.

Among the four intermediate terminals to which the voltage V3 has beenassigned in the first connection state, the voltage V3 remains to beassigned to one of the four intermediate terminals, in the secondconnection state. Then, the voltage V1 is assigned to one of the threeintermediate terminals, and the voltage V2 is assigned to two of thethree intermediate terminals, in the second connection state. To threeof the intermediate terminals to which the voltage V3 has not beenassigned (to which the voltages V1 and V2 have been assigned) in thefirst connection state, the voltage V3 is assigned in the secondconnection state.

As described above, it can be easily inferred that when an algorithm forperforming switching between the voltages and the intermediate terminalsis expanded, the present invention can be applied to a DAC as wellobtained by expanding the DACs in FIG. 2 and FIG. 12.

That is, in the first connection state, among the intermediate terminalsT1 to T(2^(M)−1), the voltage VM is assigned to 2^((M−1)) of theintermediate terminals, and the voltage V1 is assigned to one of theremainder of the intermediate terminals. The voltage V2 is assigned totwo of the remainder of the intermediate terminals. The voltage V3 isassigned to four of the remainder of the intermediate terminals. Then,the voltage V(M−1) is assigned to 2^((M−2)) of the remainder of theintermediate terminals.

In the second connection state, among the 2^((M−1)) terminals to whichthe voltage VM has been assigned in the first connection state, thevoltage VM remains to be assigned to one of the 2^((M−1)) terminals. Thevoltage V1 is assigned to one of the remaining (2^((M−1))−1)intermediate terminals. The voltage V2 is assigned to two of theremaining (2^((M−1)−)1) intermediate terminals. The voltage V3 isassigned to four of the remaining (2^((M−1))−1) intermediate terminals.The voltage V(M−1) is assigned to 2^((M−2)) intermediate terminals. Toall the (2^((M−1))−1) intermediate terminals to which the voltages V1 toV(M−1) have been assigned in the first connection state, the voltage VMis assigned.

By performing the switching as described above, the present inventioncan be applied even in a case (where (2^(M)−1) differential pairs areused in general) other than cases where three differential pairs areused and where seven differential pairs are used. When M is set to four,for example, the number of the intermediate terminals becomes 2⁴−1=15.

Among the intermediate terminals T1 to T15, the connection switch 11assigns the voltage V1 to one of the intermediate terminals. Theconnection switch 11 assigns the voltage V2 to two of the intermediateterminals. The connection switch 11 assigns the voltage V3 to four ofthe intermediate terminals. The connection switch 11 assigns the voltageV4 to eight of the intermediate terminals.

Then, by connection switching according to the present invention, thevoltage V4 is assigned to eight of the intermediate terminals T1 to T15in the first connection state. Then, the voltage V1 is assigned to oneof the remaining intermediate terminals. The voltage V2 is assigned totwo of the remaining intermediate terminals. The voltage V3 is assignedto four of the remaining intermediate terminals.

In the second connection state, switching should be performed so thatthe voltage V4 remains to be assigned to one of the eight intermediateterminals to which the voltage V4 has been assigned in the firstconnection state, the voltage V1 is assigned to one of the remainingseven intermediate terminals, the voltage V2 is assigned to two of theremaining seven intermediate terminals, the voltage V3 is assigned tofour of the remaining seven intermediate terminals, and the voltage V4is assigned to all the seven intermediate terminals to which thevoltages V1 to V3 have been assigned in the first connection state.

By applying the present invention as described above, influence ofdevice variations of the operation unit can be reduced only by switchingbetween the two connection states, and a highly accurate output circuitcan be thereby implemented.

Next, a description will be given to a configuration in which the outputcircuit of the present invention has been applied to a data driver of adisplay device for liquid crystal application. FIG. 23 is a diagramshowing a configuration of a data driver of a display device in anexample of the present invention. Though no particular limitation isimposed, FIG. 23 shows an example in which a digital-to-analog convertercircuit (DAC) 15 is formed of the connection switch 11, the operationunit 12, and the decoder 13 described in the second example. The datadriver in FIG. 23 is the data driver capable of performing a 6-bit (64gray scale) output. An output Tout of each DAC (15) is connected to adata line (indicated by reference numeral 962 in FIG. 24) of a displaypanel not shown.

Blocks of circuits such as a latch address selector 921 and a latch 922are the same as those shown in FIG. 26.

A reference voltage generation circuit (16) generates four referencevoltages (Vrefl to Vref4) for 64 output levels, and is shared by theDACs (15). Then, when the four reference voltages are set to first,eighth, fifty-seventh, and sixty-fourth voltages of 64-level voltagesthat are equally spaced, respectively, an output voltage of each DAC(15) is expressed by Expression (2), and the 64 output levels becomelinear.

Accordingly, with respect to the number of output voltages of the 64levels, the number of the reference voltages is four. Area saving of theDAC can be thereby implemented.

The connection switching signal common to the DACs (15) is supplied tothe connection switch 11, and the connection switch 11 assigns threevoltages selected by the decoder 13 to the seven intermediate terminalsof the operation unit 12.

Then, in response to the connection switching signal, each DAC (15)assumes the first connection state or the second connection state. Thus,outputs of the respective DACs are temporally averaged by periodicswitching of the connection switching signal.

Alternatively, the connection switching signal does not need to beprovided in common to all the DACs (15).

When the connection switching signal is supplied to odd-numbered DACscounted from the left and a complementary signal of the connectionswitching signal is supplied to even-numbered DACs among the DACs (15)in FIG. 23, the even-numbered DACs are brought into the secondconnection state when the odd-numbered DACs are in the first connectionstate. When the odd-numbered DACs are in the second connection state,the even-numbered DACs are brought into the first connection state. Bydoing so, output voltages of the respective DACs can be time averagedand can also be spatially averaged.

Each DAC in FIG. 23 may be formed of a plurality of blocks using 64(=2⁶) output levels as one block. In that case, in the reference voltagegeneration circuit 16 as well, the four reference voltages, the numberof which corresponds to the number of blocks, are provided.

Referring to FIG. 23, a power supply voltage for each of the referencevoltage generation circuit (gray scale voltage generation circuit) 16,decoder 13, and operation unit 12 is defined according to voltagesgenerated by the reference voltage generation circuit 16.

On the other hand, each of the latch address selector 921 and the latch922 can be set separately from the power supply voltage. In order toimplement area saving and power saving, a power supply voltage for eachof the latch address selector 921 and the latch 922 may be set to thepower supply voltage lower than the power supply voltage for each of thereference (gray scale) voltage generation circuit 16, decoder 13, andoperation unit 12. When this arrangement is made, a level shift circuit(not shown) is provided for level conversion. When the level shiftcircuit is applied to the present invention, a level shift circuit (notshown) may be preferably provided between the latch 922 and the decoders13.

FIG. 24 shows an example when the present invention has been applied toa display device. Referring to FIG. 24, a data driver 980 is the datadriver of the configuration in FIG. 23, and is set to receive 12-bitdata and output 4096 linear outputs.

When a linear output data driver is employed, by assigning gray scalevoltages that conform to a gamma characteristic of a display device(using a liquid crystal, an organic EL element, or the like) from amonga great number of linear output levels, the gray scale voltages thatconform to the gamma characteristic of the display device can be output.For this reason, the data driver includes the number of linear grayscales larger than the number of gray scales for display.

The example shown in FIG. 24 includes a data conversion table 991 forconverting L-bit data corresponding to gray scales for display into12-bit data (L<12) corresponding to linear gray scales, and a dataconversion circuit 990 that performs data conversion by referring to thedata conversion table 991.

As the data conversion table 991, the table associated with a gammacurve of the liquid crystal and characteristics for each of the colorsof R, G, B of the liquid crystal and the organic EL(Electroluminescence) is suitable.

The data conversion table 991 and the data conversion circuit 990 shouldbe configured so that 12-bit data is supplied to the data driver 980therefrom. It is simple and easy to provide the data conversion table991 and the data conversion circuit 990 so that the data conversiontable 991 and the data conversion circuit 990 are liked to a displaycontroller 950, as shown in FIG. 24.

A switching period of the connection switching signal supplied to eachconnection switch 11 of the data driver in FIG. 23 can be switched usingan integer multiple of a rewriting period of one screen (a frame period)of a display device or an integer multiple of a rewriting period of adata line (a line period). In this case, luminance of the display devicefor the same video data is averaged over the integer multiple of therewriting period of one screen. Display quality can be thereby improved.

Switching of the connection switching signal may be performed aplurality of times within one data period in which a gray scale voltagesignal is driven to a data line. In this case, if comparatively smallpositive and negative offsets are alternately supplied, the offsets arealleviated within the data line and are averaged because the data lineis a large capacitive load. With this arrangement, display quality canalso be improved.

As described above, when the present invention is applied to the displaydevice, an output voltage of each DAC in FIG. 23 or a driving voltage ofthe display device is temporally averaged. Thus, image displayunevenness caused by device variations of the operation unit 12 withinthe DAC can be reduced. As a result, high-quality image display can berealized.

By applying the DAC according to the present invention to any of suchdata drivers of display devices including display devices in accordancewith other method, cost reduction and frame formation of the displaydevices can be promoted, and at the same time, high-quality imagedisplay can be obtained.

The output circuit according to the present invention can be of courseapplied also to a display device such as an organic EL display of anactive matrix driving system that performs display by outputting avoltage signal of multi-valued levels to a data line, for example, likea liquid crystal display device.

Though FIG. 24 shows an example in which the linear output data driveris used, 12-bit video data may be input and 12-bit output voltage may beobtained without using the data conversion circuit 990. In this case,the output voltages of the reference voltage generation circuit 16should be set so that the output voltages conform to the gammacharacteristic of the display device.

The DACs explained in the examples described above are each formed ofMOS transistors. In a driving circuit of a liquid crystal displaydevice, each of the DACs may be formed of MOS transistors (TFT's) madeof polycrystal silicon. Though the examples described above showedexamples where the DACs described in the examples were applied to anintegrated circuit, the DACs can be of course applied to a discretedevice configuration as well.

The above description was given in connection with the examples. Thepresent invention, however, is not limited to the examples describedabove, and of course includes various variations and modifications thatcould be made within the scope of respective claims of the presentinvention of this application.

It should be noted that other objects, features and aspects of thepresent invention will become apparent in the entire disclosure and thatmodifications may be done without departing the gist and scope of thepresent invention as disclosed herein and claimed as appended herewith.

Also it should be noted that any combination of the disclosed and/orclaimed elements, matters and/or items may fall under the modificationsaforementioned.

1. An output circuit, comprising: a connection switch that includes first to third terminals for receiving first to third voltages, respectively, and first to seventh intermediate terminals, and that selects and supplies the first voltage, the second voltage, or the third voltage to each of said first to seventh intermediate terminals including selection of the same voltage for a plurality of the intermediate terminals, said connection switch switching assignment of the first to third voltages to said first to seventh intermediate terminals responsive to a connection switching signal; and an operation unit that receives the voltages supplied to said first to seventh intermediate terminals, and outputs to an output terminal of said output circuit a voltage obtained by performing a predetermined operation on the voltages supplied to said first to seventh intermediate terminals.
 2. The output circuit according to claim 1, wherein: said connection switch performs switching between the first connection state and the second connection state at a predetermined time interval, responsive to the connection switching signal; and said output circuit outputs a voltage obtained by performing time averaging of the output voltage of said operation unit in the first connection state and the output voltage of said operation unit in the second connection state.
 3. The output circuit according to claim 2, wherein: in the first connection state, said connection switch outputs the first voltage to said first intermediate terminal, outputs the second voltage to said second and third intermediate terminals, and outputs the third voltage to said fourth through seventh intermediate terminals, in the second connection state, said connection switch outputs the third voltage to said first through fourth intermediate terminals, outputs the second voltage to said fifth and sixth intermediate terminals, and outputs the first voltage to said seventh intermediate terminal, and said operation unit outputs to said output terminal an average voltage of the voltages supplied to said first to seventh intermediate terminals.
 4. The output circuit according to claim 3, wherein said connection switch includes: a first switch connected between a first terminal with the first voltage applied thereto and said first intermediate terminal; a second switch connected between a second terminal with the second voltage applied thereto and said second intermediate terminal; a third switch connected between the second terminal and said third intermediate terminal; a fourth switch connected between said third terminal with the third voltage applied thereto, and said fifth intermediate terminal; a fifth switch connected between said third terminal and said sixth intermediate terminal; a sixth switch connected between said third terminal and said seventh intermediate terminal, said first to sixth switches being on/off controlled by the connection switching signal; a seventh switch connected between said first terminal and said seventh intermediate terminal; an eighth switch connected between said second terminal and said fifth intermediate terminal; a ninth switch connected between said second terminal and said sixth intermediate terminal; a tenth switch connected between said third terminal and said first intermediate terminal; an eleventh switch connected between said third terminal and said second intermediate terminal; and a twelfth switch connected between said third terminal and said third intermediate terminal, said seventh to twelfth switches being on/off controlled by a complementary signal of the connection switching signal, said third terminal being connected to said fourth intermediate terminal, and the third voltage being output to said fourth intermediate terminal irrespective of a state of the connection switching signal.
 5. The output circuit according to claim 2, wherein: said operation unit comprises a differential amplifier and a polarity switch;— said differential amplifier includes: first to seventh differential pairs; first to seventh current sources that supply currents to said first to seventh differential pairs, respectively; a load circuit connected in common to output pairs of said first to seventh differential pairs, first outputs of output pairs of said first to seventh differential pairs being connected in common to a first connection node, and second outputs of output pairs of said first to seventh differential pairs being connected in common to a second connection node; an amplification stage having an output terminal thereof connected to said output terminal of said output circuit; and a switching circuit that connects said first connection node or said second connection node to an input terminal of said amplification stage responsive to a predetermined control signal; said polarity switch performs switching between a first connection state and a second connection state responsive to the control signal; in the first connection state, said first to seventh intermediate terminals of said connection switch are connected to first inputs of respective input pairs of said first to seventh differential pairs, respectively, and said output terminal of said differential amplifier is connected to second inputs of said respective input pairs of said first to seventh differential pairs; and in the second connection state, said output terminal of said differential amplifier is connected to the first inputs of said respective input pairs of said first to seventh differential pairs, and said first to seventh intermediate terminals of said connection switch are connected to the second inputs of said respective input pairs of said first to seventh differential pairs, respectively.
 6. The output circuit according to claim 5, wherein in said differential amplifier, sizes of transistors forming said first to seventh differential pairs are set to be equal to one another; and current values of said first to seventh current sources are set to be equal to one another.
 7. The output circuit according to claim 5, wherein the connection switching signal that controls said connection switch and the control signal that controls said polarity switch are identical.
 8. The output circuit according to claim 3, wherein in said connection switch, a plurality of said switches controlled by a same connection switching signal and having a same input voltage applied thereto are omitted, except one of the plurality of said switches.
 9. The output circuit according to claim 7, wherein said first to seventh intermediate terminals of said connection switch are divided into two groups, i.e., a first group connected to a first input of each input pair of the first to seventh differential pairs, respectively, and a second group connected to a second input of each input pair of the first to seventh differential pairs, respectively; and in response to the connection switching signal, in the first connection state, the first through third voltages are output to the first to seventh intermediate terminals of the first group; and in the second connection state, the first and to third voltages are output to the first to seventh intermediate terminals of the second group.
 10. A data driver that drives data lines based on an input digital data signal, comprising the output circuit as set forth in claim
 2. 